In the pulp and paper industry the so-called black liquor is sprayed into a recovery boiler for recovery of the spent chemicals used in Kraft process pulping or delignification and for recovery of energy from waste organics also contained in the black liquor. The droplet size of black liquor sprayed in the Kraft pulping process recovery boiler is critical. If the droplets are too large all of the water does not evaporate and water may enter the char bed in the lower part of the furnace and shift the char bed chemistry in an undesirable direction, e.g. toward production of hydrogen sulphide. If the droplet size is too small the residual sulfur and sodium may coat the tubes at the top of the furnace and have to be removed with steam. At the critical size range the residual inorganic residues fall to the bottom of the furnace where they can be recovered. Ordered droplet control within the critical size range greatly improves the efficiency and reduces the cost of Kraft process chemical and energy recovery.
In high throughput or high flow rate nozzles control of droplet size is difficult because of the turbulent and chaotic flow conditions in the nozzle. At these high production rates the surface tension effects of the liquid are overwhelmed by shear forces resulting in chaotic droplet formation in conventional nozzles. This is typical in power boiler nozzles where spray atomization is used for efficient fuel combustion. There is no concern for ordered droplet formation or droplet uniformity. The objective of spray atomization is to achieve the largest possible liquid surface area and it is typically used in fuel combustion and fuel injection.
The present invention is also to be contrasted with nozzle applications requiring low flow rates in gentle flow environments where surface tension may be used to control droplet size. Ordered droplet production and uniform size control of spray from nozzles may be important in a variety of applications, for example, some types of spray drying, agricultural spraying, ink jet printing, cooling tower spray cooling, and freeze drying. Some of these applications, however, are not concerned with high throughput and high volume flow rate, and low flow rates may facilitate control of droplet size.